c++: Implement __is_{nothrow_,}convertible [PR106784]

c++: Implement __is_{nothrow_,}convertible [PR106784]

To improve compile times, the C++ library could use compiler built-ins
rather than implementing std::is_convertible (and _nothrow) as class
templates.  This patch adds the built-ins.  We already have
__is_constructible and __is_assignable, and the nothrow forms of those.

Microsoft (and clang, for compatibility) also provide an alias called
__is_convertible_to.  I did not add it, but it would be trivial to do
so.

I noticed that our __is_assignable doesn't implement the "Access checks
are performed as if from a context unrelated to either type" requirement,
therefore std::is_assignable / __is_assignable give two different results
here:

  class S {
    operator int();
    friend void g(); // #1
  };

  void
  g ()
  {
    // #1 doesn't matter
    static_assert(std::is_assignable<int&, S>::value, "");
    static_assert(__is_assignable(int&, S), "");
  }

This is not a problem if __is_assignable is not meant to be used by
the users.

This patch doesn't make libstdc++ use the new built-ins, but I had to
rename a class otherwise its name would clash with the new built-in.

	PR c++/106784

gcc/c-family/ChangeLog:

	* c-common.cc (c_common_reswords): Add __is_convertible and
	__is_nothrow_convertible.
	* c-common.h (enum rid): Add RID_IS_CONVERTIBLE and
	RID_IS_NOTHROW_CONVERTIBLE.

gcc/cp/ChangeLog:

	* constraint.cc (diagnose_trait_expr): Handle CPTK_IS_CONVERTIBLE
	and CPTK_IS_NOTHROW_CONVERTIBLE.
	* cp-objcp-common.cc (names_builtin_p): Handle RID_IS_CONVERTIBLE
	RID_IS_NOTHROW_CONVERTIBLE.
	* cp-tree.h (enum cp_trait_kind): Add CPTK_IS_CONVERTIBLE and
	CPTK_IS_NOTHROW_CONVERTIBLE.
	(is_convertible): Declare.
	(is_nothrow_convertible): Likewise.
	* cxx-pretty-print.cc (pp_cxx_trait_expression): Handle
	CPTK_IS_CONVERTIBLE and CPTK_IS_NOTHROW_CONVERTIBLE.
	* method.cc (is_convertible): New.
	(is_nothrow_convertible): Likewise.
	* parser.cc (cp_parser_primary_expression): Handle RID_IS_CONVERTIBLE
	and RID_IS_NOTHROW_CONVERTIBLE.
	(cp_parser_trait_expr): Likewise.
	* semantics.cc (trait_expr_value): Handle CPTK_IS_CONVERTIBLE and
	CPTK_IS_NOTHROW_CONVERTIBLE.
	(finish_trait_expr): Likewise.

libstdc++-v3/ChangeLog:

	* include/std/type_traits: Rename __is_nothrow_convertible to
	__is_nothrow_convertible_lib.
	* testsuite/20_util/is_nothrow_convertible/value_ext.cc: Likewise.

gcc/testsuite/ChangeLog:

	* g++.dg/ext/has-builtin-1.C: Enhance to test __is_convertible and
	__is_nothrow_convertible.
	* g++.dg/ext/is_convertible1.C: New test.
	* g++.dg/ext/is_convertible2.C: New test.
	* g++.dg/ext/is_nothrow_convertible1.C: New test.
	* g++.dg/ext/is_nothrow_convertible2.C: New test.